Background: The aim of this study was to investigate the performance and predictive value of hypocalcemia in severe COVID-19 patients. Methods: We retrospectively investigated the clinical and laboratory characteristics of severe COVID-19 patients. 107 patients were divided into hypocalcemia group and normal serum calcium group. The clinical and laboratory data were compared between two groups. The discriminative power of hypocalcemia regarding poor outcome were evaluated by receiver operating curves (ROC) analyses. Results: Sixty seven patients (62.6%) had hypocalcemia. In hypocalcemia group, leukocytes, c-reactive protein (CRP), procalcitonin (PCT), Interleukin 6 (IL-6), and D-dimer levels was higher, while lymphocytes and albumin (ALB) levels was lower. No significant difference was identified in gender, age, signs and symptoms, comorbidities and other laboratory indicators. Serum calcium levels were negatively correlated with leukocytes, CRP, PCT, IL-6 and D-dimer, while positively correlated with lymphocytes and ALB. Patients with hypocalcemia more commonly presented poor outcome (47.8% (32/67) vs 25% (10/40), p = 0.02). Median serum calcium levels were significantly lower in the patients with poor outcome (2.01(1.97-2.05) vs 2.10(2.03-2.20), p < 0.001), and it could predict the prognosis with an area under the ROC curve (AUC) of 0.73(95% confidence interval (CI) 0.63-0.83, p < 0.001). Conclusions: Hypocalcemia commonly occurred in severe COVID-19 patients and it was associated with poor outcome. (C) 2020 The Author(s). Published by Elsevier Ltd on behalf of King Saud Bin Abdulaziz University for Health Sciences.